This invention relates to motor controls, and in particular, to a method of controlling the starting of an AC induction motor with a soft starter.
There are two basic approaches for controlling the starting, stopping and speed of an AC induction motor. In a first approach, an adjustable frequency controller is interconnected to the AC induction motor. The adjustable frequency controller is comprised of an inverter which uses solid state switches to convery DC power to stepped waveform AC power. A waveform generator produces switching signals for the inverter under control of a microprocessor. While adjustable frequency controllers efficiently control the motor speed and the energy used by an AC induction motor, use of such types of controllers may be cost prohibitive. Further, since many applications of AC induction motors do not require sophisticated frequency and voltage control, an alternative to adjustable frequency controllers has been developed.
An alternate approach to the adjustable frequency controller is the soft starter. Soft starters operate using the principal of phase control whereby the line currents supplied to the AC induction motor are controlled by means of anti-parallel thyristor switches. Soft starters offer two major benefits for a user. First, use of a soft starter reduces the motor torque pulsation at startup of the AC induction motor which, in turn, results in less mechanical strain on the load. Second, use of a soft starter reduces the motor inrush current at startup of the AC induction motor which, in turn, places less stress on upstream electrical systems.
Typically, in a soft starter, the anti-parallel thyristor switches are provided in each supply line. These thyristor switches in each supply line are fired to control the fraction of the half cycle over which current is conducted to the motor (known as the conduction period). The non-conducting period of each half cycle (known as the hold-off angle or the notch width) is visible as a notch in the voltage waveform at each motor terminal. During this period, no current flows to the motor terminals. To end the non-conducting period, the thyristor switches in the supply line to the motor terminals are fired to restart their conduction. The conduction through the thyristor switches continues until the current, once again, becomes zero at some point in the next half cycle and the thyristor switches reopen. According to the principles of phase control, by varying the duration of the non-conducting period, the voltage and current supplied to the AC induction motor may be controlled.
Alternatively, with delta motors, the anti-parallel thyristor switches may be provided inside the delta. Positioning the anti-parallel thyristor switches within the delta allows for use of smaller electrical components since the phase current magnitudes are less than the line current magnitudes.
Heretofore, in order to start an AC induction motor using a soft starter, two alternate types of control algorithms are used. In alpha control, the thyristor switches are sequentially fired at a certain angle, alpha (xcex1), after the zero crossing times of the corresponding phase voltages. In gamma control, the thyristor switches are sequentially fired at a certain angle, gamma (xcex3), after the zero crossing times of the corresponding phase currents. It has been found that alpha control is more stable than gamma control in starting and bringing the AC induction motor to near operating speed because the zero crossings of the phase voltage are fixed while the zero crossings of the phase current move as the speed of the AC induction motor changes.
In addition, if gamma control is used during such period, greater current oscillation occurs which, in turn, leads to greater torque oscillation. More specifically, torque oscillation when the motor is stalled or when accelerating up to speed generally is caused by a DC component in the motor current. One of the principle advantages of utilizing a soft starter is that by applying the voltage to the motor slowly, these DC offset currents can be eliminated. However, it is possible to re-introduce DC currents when using gamma control. For example, utilizing a constant gamma when the integral of the motor phase current for the positive half cycle differs from the integral of the motor phase current for the negative half cycle, produces a DC component. In fact, if a disturbance introduces a slight asymmetry in firing of the thyristor switches, gamma control will exhibit a tendency to amplify the asymmetry, since an early current zero crossing will cause the next thyristor firing to occur earlier with respect to the terminal voltage. This effect is more pronounced when the soft starter is positioned inside the delta because the thyristor switches directly control phase voltage, that the current through the thyristor switches reflects the actual power of the motor (mostly inductive during acceleration), whereas line current control interposes a 30 degree angle between motor terminal voltage and line current being controlled.
Alternatively, gamma control is preferred once the motor has accelerated to the point where its speed is above the breakdown speed, in other words, the speed at which the slope of the speed/torque curve is zero. This is due to the fact that the motor""s torque production is a delayed function of speed. Therefore, as the motor accelerates to full speed, it overshoots before the electrical torque balances the mechanical load. Once the motor overshoots, the electrical torque eventually becomes negative and the speed undershoots its steady state value causing torque to become positive again. It may take several cycles of this oscillation to reach steady state when the terminal voltage on the motor is fixed. However, when gamma control is used, the changes in motor power factor caused by the speed changes result in changes in current zero cross time which, in turn, affect the firing point for the next half cycle of current. This results in slight cycle to cycle changes in terminal voltage. These changes have a damping effect on the oscillations described above. In fact, in many cases, the oscillations are damped within a complete cycle.
Therefore, it is a primary object and feature of the present invention to provide an improved method for controlling the starting of an AC induction motor.
It is a further object and feature of the present invention to provide a method for controlling the starting of an AC induction in a stable manner.
It is still a further object and feature of the present invention to provide a method for controlling the starting an AC induction motor which is simple and less expensive than prior art methods.
In accordance with the present invention, a method is provided for controlling a three-phase AC induction motor. Each phase of the induction motor is interconnected to an AC input source by a thyristor switch in order to provide voltage and current to the AC induction motor. The method includes the steps of sequentially firing pairs of thyristor switches at a predetermined initial gamma firing angle after opening of the thyristor switches. Each thyristor switch opens in response to zero current therethrough. A new gamma firing angle is calculated which is dependant on the initial gamma firing angle. Pairs of thyristor switches are sequentially fired at the new gamma firing angle after opening of the thyristor switch.
Bypass contactors may be provided in parallel across corresponding thyristor switches. The operating speed of the AC induction motor is monitored and the bypass contactors are closed in response to the AC induction motor operating at a predetermined operating speed. If the operating speed is less than the predetermined operating speed for the AC induction motor, the new gamma firing angle is provided as the initial gamma firing angle. Thereafter, a new gamma firing angle is calculated according to the expression:
xcex3i=xcex3ixe2x88x921+k*(Ixe2x88x92Ilim)
wherein xcex3i is the new gamma firing angle; xcex3ixe2x88x921 is the initial gamma firing angle; k is a proportional gain; I is an integral of the motor current over a conduction time interval; and Ilim is a user selected pre-set current limit. The absolute value of the expression k* (Ixe2x88x92Ilim) is compared to a gamma limit prior to calculating the new gamma firing angle. The expression k*(Ixe2x88x92Ilim) is replaced with the gamma limit in the expression to calculate the new gamma firing angle if the absolute value of the expression k*(Ixe2x88x92Ilim) is greater than the gamma limit.
In accordance with a still further aspect of the present invention, a method is provided for controlling the voltage and current supplied to an AC motor. The AC motor is interconnected to an AC input source via a thyristor switch. The method includes the steps of initially firing the thyristor switch at a predetermined gamma firing angle after opening of the thyristor switch. The predetermined gamma firing angle is provided as an initial gamma firing angle and the thyristor switch is opened in response to zero supply current therethrough. A new gamma firing angle, dependant upon the initial gamma firing angle, is calculated and the thyristor switch is fired at the new gamma firing angle after occurrence of zero supply current therethrough. Thereafter, the new gamma firing angle is provided as the initial gamma firing angle.
It is contemplated to provide a bypass contactor in parallel to the thyristor switch. The operating speed of the AC motor is monitored and the bypass switch is closed in response to the AC motor operating at a predetermined operating speed. If the operating speed is less than the predetermined operating speed for the AC motor, a new gamma firing angle is calculated. The new gamma firing angle is according to the expression:
xcex3i=xcex3ixe2x88x921+k*(Ixe2x88x92Ilim)
wherein xcex3i is the new gamma firing angle; xcex3ixe2x88x921 is the initial gamma firing angle; k is a proportional gain; I is an integral of the motor current over a conduction time interval; and Ilim is a user selected pre-set current limit. The absolute value of the expression k* (Ixe2x88x92Ilim) is compared to a gamma limit prior to calculating the new gamma firing angle. The expression k*(Ixe2x88x92Ilim) is replaced with the gamma limit in the expression to calculate the new gamma firing angle if the absolute value of the expression k*(Ixe2x88x92Ilim) is greater than the gamma limit.
In accordance with a still further aspect of the present invention, a method is provided for controlling a three-phase, AC induction motor. Each phase of the AC induction motor is interconnected to an AC source by a thyristor switch and a bypass contactor connected in parallel in order to provide voltage and current to the AC induction motor. The method includes the steps of sequentially firing pairs of thyristor switches at a predetermined initial gamma firing angle after opening of the thyristor switches. Each thyristor switch opens in response to zero supply current therethrough. A new gamma firing angle is calculated which is dependant upon the difference between an integral of motor current over a conduction time interval and a user selected pre-set current limit. Pairs of thyristor switches are fired at the new gamma firing angle after opening of the thyristor switches. The operating speed of the AC induction motor is monitored and if the operating speed is less than a predetermined operating speed, the new gamma firing angle is provided as the initial gamma firing angle, and a new gamma firing angle is calculated. The new gamma firing angle is according to the expression:
xcex3i=xcex3ixe2x88x921+k*(Ixe2x88x92Ilim)
wherein xcex3i is the new gamma firing angle; xcex3ixe2x88x921 is the initial gamma firing angle; k is a proportional gain; I is the integral of the motor current over a conduction time interval; and Ilim is the user selected pre-set current limit. The absolute value of the expression k *(Ixe2x88x92Ilim) is compared to a gamma limit prior to calculating the new gamma firing angle. The expression k*(Ixe2x88x92Ilim) is replaced with the gamma limit in the expression to calculate the new gamma firing angle if the absolute value of the expression k*(Ixe2x88x92Ilim) is greater than the gamma limit. It is contemplated to close the bypass contactors in response to the AC motor operating at the predetermined operating speed.